1. Field of the Invention
The present invention relates to a polishing progress motoring method and a polishing apparatus, and more particularly to a polishing progress motoring method and a polishing apparatus for monitoring a change in thickness of a transparent insulating film during polishing of the film.
The present invention also relates to a method and an apparatus for selecting wavelengths of light for use in an optical polishing end point detection of a substrate having a transparent insulating film.
The present invention also relates to a method and an apparatus for detecting a polishing end point of a substrate having an insulating film, and more particularly to a method and an apparatus for detecting a polishing end point based on reflected light from a substrate. The present invention also relates to a polishing method and a polishing apparatus for polishing a substrate while monitoring reflected light from the substrate.
The present invention also relates to a polishing method and a polishing apparatus for a substrate using an optical polishing end point detection unit, and more particularly to a polishing method and a polishing apparatus suitable for use in identifying a cause of photocorrosion of a metal film.
The present invention also relates to a method of monitoring a polishing process of a substrate having an insulating film, and more particularly to a method of monitoring a polishing process of a substrate based on reflected light from the substrate.
2. Description of the Related Art
In fabrication processes of a semiconductor device, several kinds of materials are repeatedly deposited as films on a silicon wafer to form a multilayer structure. To realize such a multilayer structure, it is important to planarize a surface of a top layer. A polishing apparatus for performing chemical mechanical polishing (CMP) is used as one of techniques for achieving such planarization.
The polishing apparatus of this type includes, typically, a polishing table supporting a polishing pad thereon, a top ring for holding a substrate (a wafer with a film formed thereon), and a polishing liquid supply mechanism for supplying a polishing liquid onto the polishing pad. Polishing of a substrate is performed as follows. The top ring presses the substrate against the polishing pad, while the polishing liquid supply mechanism supplies the polishing liquid onto the polishing pad. In this state, the top ring and the polishing table are moved relative to each other to polish the substrate, thereby planarizing the film of the substrate. The polishing apparatus typically includes a polishing end point detection unit. This polishing end point detection unit is configured to determine a polishing end point based on a time when the film is removed to reach a predetermined thickness or when the film in its entirety is removed.
One example of such polishing end point detection unit is a so-called optical polishing end point detection apparatus, which is configured to apply light to a surface of a substrate and determine a polishing end point based on information contained in reflected light from the substrate. The optical polishing end point detection apparatus typically includes a light-applying section, a light-receiving section, and a spectroscope. The spectroscope decomposes the reflected light from the substrate according to wavelength and measures reflection intensity at each wavelength. This optical polishing end point detection apparatus is often used in polishing of a substrate having a light-transmittable film. For example, the Japanese laid-open patent publication No. 2004-154928 discloses a method in which intensity of reflected light from a substrate (i.e., reflection intensity) is subjected to certain processes for removing noise components to create a characteristic value and the polishing end point is detected from a distinctive point (a local maximum point or a local minimum point) of the temporal variation in the characteristic value.
The characteristic value created from the reflection intensity varies periodically with a polishing time as shown in FIG. 1, and local maximum points and local minimum points appear alternately. This phenomenon is due to interference between light waves. Specifically, the light, applied to the substrate, is reflected off an interface between a medium and a film and an interface between the film and an underlying layer. The light waves from these interfaces interfere with each other. The manner of interference between the light waves varies depending on the thickness of the film (i.e., a length of an optical path). Therefore, the intensity of the reflected light from the substrate (i.e., the reflection intensity) varies periodically in accordance with the thickness of the film. The reflection intensity can also be expressed as a reflectance.
The above-described optical polishing end point detection apparatus counts the number of distinctive points (i.e., the local maximum points or local minimum points) of the variation in the characteristic value after the polishing process is started, and detects a point of time when the number of distinctive points has reached a preset value. Then, the polishing process is stopped after a predetermined period of time has elapsed from the detected point of time.
The characteristic value is an index (a spectral index) obtained based on the reflection intensity measured at each wavelength. Specifically, the characteristic value is given by the following equation (1):Characteristic value(Spectral Index)=ref(λ1)/(ref(λ1)+ref(λ2)+ . . . +ref(λk))  (1)
In this equation (1), λ represents a wavelength of the light, and ref (λk) represents a reflection intensity at a wavelength λk. The number of wavelengths λ to be used in calculation of the characteristic value is preferably two or three (i.e., k=2 or 3).
As can be seen from the equation (1), the reflection intensity is divided by the refection intensity. This process can remove noise components contained in the reflection intensity (i.e., noise components generated by the increase and decrease in the amount of reflected light regardless of the wavelength). Therefore, the characteristic value with less noise components can be obtained. Instead of the characteristic value, the reflection intensity (or reflectance) itself may be monitored. In this case also, since the reflection intensity varies periodically according to the polishing time in the same manner as the graph shown in FIG. 1, the polishing end point can be detected based on the change in the reflection intensity.
Further, the characteristic value may be calculated using relative reflectance that is created based on the reflection intensity. The relative reflectance is a ratio of an actual intensity of reflected light (which is determined by subtracting a background intensity from a reflection intensity measured) to a reference intensity of light (which is determined by subtracting the background intensity from a reference reflection intensity). The background intensity is an intensity that is measured under conditions where no reflecting object exists. The relative reflectance is determined by subtracting the background intensity from both the reflection intensity at each wavelength during polishing of the substrate and the reference reflection intensity at each wavelength that is obtained under predetermined polishing conditions to determine the actual intensity and the reference intensity and then dividing the actual intensity by the reference intensity. More specifically, the relative reflectance is obtained by usingthe relative reflectance R(λ)=[E(λ)−D(λ)]/[B(λ)−D(λ)]  (2)where λ is a wavelength, E(λ) is a reflection intensity with respect to a substrate as an object to be polished, B(λ) is the reference reflection intensity, and D(λ) is the background intensity (dark level) obtained under conditions where the substrate does not exist or the light from a light source toward the substrate is cut off by a shutter or the like. The reference reflection intensity B(λ) may be an intensity of reflected light from a silicon wafer when water-polishing the silicon wafer while supplying pure water onto the polishing pad. In this specification, the reflection intensity and the relative reflectance will be collectively referred to as reflection intensity.
Using relative reflectances determined from the equation (2), the characteristic value can be calculated from the following equation (3):The characteristic value S(λ1)=R(λ1)/(R(λ1)+R(λ2)+ . . . +R(λk))  (3)
In this equation, λ is a wavelength of light, and R(λk) is a relative reflectance at a wavelength λk. The number of wavelengths λ to be used in calculation of the characteristic value is preferably two or three (i.e., k=2 or 3).
Further, using the above-described relative reflectances at plural wavelengths λk (k=1, . . . , K) and weight functions, the characteristic value S (λ1, λ2, . . . , λK) may be calculated from the following equations:X(λk)=∫R(λ)·Wk(λ)dλ  (4)The characteristic value S(λ1, λ2, . . . , λK)=X(λ1)/[X(λ1)+X(λ2)+ . . . +X(λK)]=X(λ1)/ΣX(λk)  (5)
In the above equation (4), Wk(λ) is a weight function having its center on the wavelength λk (i.e., a weight function having its maximum value at the wavelength λk). FIG. 2 shows examples of the weight function. The maximum value and the width of the weight function shown in FIG. 2 can be changed appropriately. In the equation (4), interval of integration is from a minimum wavelength to a maximum wavelength of a measurable range of the optical polishing end point detection apparatus. For example, where the optical polishing end point detection apparatus has its measurable range from 400 nm to 800 nm, the interval of integration is from 400 to 800.
The above-described optical polishing end point detection apparatus counts the number of distinctive points (i.e., the local maximum points or local minimum points) of the variation in the characteristic value which appear after the polishing process is started as shown in FIG. 1, and determines a time when the number of distinctive points reaches a preset number. Then, the polishing process is stopped after a predetermined period of time has elapsed from the determined time. However, in this polishing end point detection method, when the thickness of the film to be removed (i.e., an amount of film to be removed) is small, only one or two distinctive points appear during polishing even if the wavelengths are appropriately selected. This makes it difficult to monitor the progress of the polishing process.
If light with a shorter wavelength is used, a larger number of distinctive points are expected to appear. However, application of light with a short wavelength to a substrate can cause a problem of so-called photocorrosion. This photocorrosion is a phenomenon of corrosion that occurs in interconnect metal, such as copper, as a result of application of light thereto. In addition, in a case where light with a short wavelength in ultraviolet region is used, a normal glass material cannot be used in an optical transmission system, and as such quartz is needed. Moreover, a dedicated light source and a dedicated spectroscope are needed, thus increasing a cost of the apparatus.
Further, as shown in FIG. 3, an underlying layer generally has a surface with convex and concave portions. Due to a variation in size of the convex and concave portions, appearance times of the local maximum points and the local minimum points of the characteristic value may vary from substrate to substrate. For example, as shown in FIG. 4, when polishing a film having initial thicknesses of 400 nm and 750 nm, a local maximum point of the characteristic value appears at a certain point of time that is different from that in the case of polishing a film having initial thicknesses of 400 nm and 785 nm, even if a removal rate is the same. Consequently, the resultant thickness of the polished film varies from substrate to substrate, and a yield of products is lowered.
In particular, in a process of polishing a layer composed of a copper interconnect material and an insulating material after removing a copper film and a barrier film, it is necessary to accurately detect the polishing end point. The purpose of this polishing process is to adjust a height of the interconnects (i.e., an ohmic value or resistance) by polishing the layer composed of the copper interconnect material and the insulating material after removing the copper film (i.e., the interconnect material) and the underlying barrier film (e.g., tantalum or tantalum nitride). If an accurate polishing end point detection is not performed in this polishing process, the ohmic value of the interconnects varies greatly. Thus, in this polishing process, shift of the appearance times of the local maximum points and the local minimum points due to the variation in the initial film thickness including the underlying layer is not permitted from the viewpoint of the required accuracy. In addition, it is necessary to avoid the influence of the photocorrosion on the interconnects.
To detect an accurate polishing end point, it is necessary to select the wavelengths such that a local maximum point or a local minimum point of the characteristic value appears when the film thickness approaches or reaches a target thickness. However, in actual procedures, the optimum wavelengths are found by trial and error, and hence a long time is needed to select the wavelengths.
In a polishing process for the purpose of exposing a lower film by polishing an upper film, e.g., a polishing process for STI (shallow trench isolation) formation, it is customary to adjust a polishing liquid such that a polishing rate of the lower film is lower than that of the upper film. This is for preventing excess-polishing of the lower film to stabilize the polishing process. However, when the polishing rate is low, the characteristic value (or the reflection intensity) does not fluctuate greatly, as shown in FIG. 5. As a result, the periodical variation in the characteristic value is hardly observed and it is therefore difficult to detect the distinctive point (the local maximum point or local minimum point) of the characteristic value. Consequently, an accurate polishing end point detection cannot be achieved. In addition, since the fluctuation of the characteristic value (or the reflection intensity) is affected by the thickness of both the upper film and the lower film and the type of films, the difference in the initial film thickness between substrates may cause an error of the polishing end point detection. Generally, the difference in the initial film thickness between substrates in each process lot is about ±10%. Such a variation in the initial film thickness can result in an error of the polishing end point detection, because even if the distinctive point (the local maximum point or local minimum point) of the characteristic value is detected, a relationship between the distinctive point of the characteristic value (or the reflection intensity) and the exposure point of the lower film may be altered due to the difference in the film thickness between substrates.
FIG. 6 is a cross-sectional view showing a multilayer interconnect structure formed on a silicon wafer. An oxide film 100 having a gate structure is formed on the silicon wafer. Multiple SiCN films 101 and oxide films (e.g., SiO2) 102 are formed on the oxide film 100. The oxide films 102 function as an inter-level dielectric, and the SiCN films 101 function as an etch stopper and a diffusion-preventing layer for the inter-level dielectric. A trench 103 and a via plug 104 are formed in the oxide films 102. A barrier film (e.g., TaN, Ta, Ru, Ti, or TiN) 105 is formed on surfaces of the trench 103 and the via plug 104 and an upper surface of the oxide film 102. Further, a copper film M2 is formed on the barrier film 105, so that the trench 103 and the via plug 104 are filled with part of the copper film M2. The trench 103 is formed according to interconnect patterns, and the copper filling the trench 103 provides metal interconnects. The copper in the trench 103 is electrically connected to lower-level copper interconnects M1 via the copper in the via plug 104.
The copper film M2 formed on areas, other than the trench 103 and the via plug 104, is an unnecessary copper film which causes short circuit between the interconnects. This unnecessary copper film is polished by the above-described polishing apparatus. As shown in FIG. 6, polishing of the copper film M2 is performed in approximately two steps. The first step is a process of removing the exposed copper film M2. In this first step, only the copper film M2, which is metal, is polished. Therefore, an eddy current sensor is used to monitor the progress of polishing of the copper film M2. The second step is a process of removing the barrier film 105 after the exposed copper film M2 is removed and then polishing the copper in the trench 103, together with the oxide film 102. Removal of the barrier film 105 can be detected by an eddy current sensor or a table-current sensor (which measures a change in current of a motor rotating the polishing table caused in response to a change in frictional torque between the surface of the substrate and the polishing pad). When the barrier film 105 is thin enough to allow the light to pass therethrough, it is possible to detect the removal of the barrier film 105 by the optical polishing end point detection apparatus. Because the height of the copper in the trench 103 determines the resistance of the interconnects, it is important to accurately detect the polishing end point in the second step. As can be seen from FIG. 6, in the second step, the oxide film 102 is mainly polished. Therefore, the optical polishing end point detection apparatus is used to monitor the progress of polishing in the second step.
As described above, the optical polishing end point detection apparatus is suitable for use in polishing of a light-transmittable film, such as an oxide film. However, when the optical polishing end point detection apparatus is used in polishing of a metal film, such as a copper film, the photocorrosion can occur in the metal film. The photocorrosion is a phenomenon of corrosion of a material caused by application of light thereto. Specifically, when light is applied to the material, photoelectromotive force is generated in the material to produce an electric current that flows therethrough, causing corrosion of the material. This photocorrosion can cause a change in resistance of the metal interconnects, thus causing defects of a semiconductor device as a product. Accordingly, preventing the photocorrosion is one of the important issues in the fabrication process of the semiconductor device.
It is considered that the photocorrosion is likely to occur in the presence of a liquid. Since the polishing liquid is used in polishing of a substrate, it is important to prevent the photocorrosion during polishing of the substrate. Generally, the photocorrosion is considered to occur depending on illuminance of light (expressed by “lux”). However, most of detailed conditions where the photocorrosion occurs are unknown. As a result, it is still difficult to prevent the photocorrosion from occurring.
The characteristic value as shown in FIG. 1 fluctuates periodically according to the thickness of the light-transmittable film which is reduced as the polishing process proceeds. Therefore, the characteristic value can be regarded as an index that indicates the progress of polishing of the film. However, the substrate generally has a multilayer structure composed of metal interconnects with different patterns and multiple insulating films having light transmission characteristics. Therefore, the optical polishing end point detection apparatus detects a film thickness that reflects not only an uppermost insulating film, but also an underlying insulating film. For example, in an example shown in FIG. 7, a lower insulating film is formed on a silicon wafer, and a metal interconnect and an upper insulating film are formed on the lower insulating film. A thickness to be monitored during polishing is a thickness of the upper insulating film. However, part of the light emitted from the optical polishing end point detection apparatus travels through the upper insulating film and the lower insulating film and reflects off underlying metal interconnects, elements with no light transmission characteristic, and the silicon wafer. As a result, the characteristic value calculated by the optical polishing end point detection apparatus reflects both the thickness of the upper insulating film and the thickness of the lower insulating film. In this case, if the thickness of the lower insulating film varies from region to region (as indicated by d1 and d2 in FIG. 7), a reliable characteristic value cannot be obtained, and hence the accuracy of the polishing end point detection is lowered. In addition, even if substrates have the same structure, the thickness of the lower insulating film may vary from substrate to substrate. In this case also, the accuracy of the polishing end point detection is lowered.